Page 158 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 151

100

%T
Long-pass filters (clear and pale yellow)
50
WG280 WG320 WG345 GG375 GG395 GG420 GG435 GG455

300 l (nm) 400 500
100

%T
Long-pass filters (yellow, orange, red)
50
GG475 GG495 OG515 OG530 OG550 OG570 OG590 RG610 RG630 RG645

500 l (nm) 600 700
100

%T
Long-pass filters (black IR transmitting)
50
RG665 RG695 RG715 RG780 RG850 RG1000

800 l (nm) 1000 1200
Figure 7.10 Examples of colored glass filter responses.
Edge filters are available with modest slopes and very
strong blocking of out-of-band wavelengths. These
are very useful for many instrument applications.
Figure supplied and reproduced by permission of
Comar Ltd, Cambridge U.K.

exist in different valencies and oxidation states, giving a range of absorption
spectra for a single ion. Several can even be switched between states via the
solution pH. As pH can be electrically controlled, so can the optical filter
response.
Gases can also be useful in a few situations. Bromine, chlorine, iodine, and
sulphur dioxide have strong absorptions in the 300 to 600nm range, adjustable
via their vapor pressure, or by their concentration when dissolved in water or
carbon tetrachloride. Even if the absorption spectrum is highly complex, its
strength can be tuned using pressure. This approach can be used to make an

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